The timing gear plays a key role in each piston engine as it controls the opening and closing of the valves, letting in a mix of air and fuel and letting out exhaust gases. The timing gear is driven by a belt, rarely by a chain. If the belt breaks, the valves and pistons may collide, causing a costly damage. Therefore, the timing gear components, i.e. the belt or chain, tensioning rollers and guide rollers should regularly be changed and checked for any signs of wear.